The invention is directed to the determination of a relative position of two objects with respect to one another.
A robot is being increasingly employed for assembly work due to electronic and electro-technical progress. This robot is equipped with a flexible gripper system that is in the position of implementing the greatest variety of handling and manipulation tasks.
Document [1] provides an overview of technical systems that are utilized for grasping and manipulating an object. Further, [1] discloses that the grasped object must be in a permanently prescribed position relative to the gripper system, namely such that a prescribable force can be exerted on the object.
It is known from [1] that manipulation means that the position of the grasped object and, potentially, its orientation in space is modified in a predetermined way.
In conjunction with a gripper system, it thereby turns out that the determination of the relative position of the grasped object with respect to the gripper and the force exerted on the grasped object are of particular significance.
It is also known from [1] that what is referred to as an object and contact sensor is employed for determining the position and the force.
What is understood by an object sensor is a sensor that acquires the position of an object. With such a sensor, the position of an object is determined, a known object is recognized and identified and an unknown object is measured.
A typical implementation of this sensor is an ultrasound distance sensor, a laser distance sensor or a camera viewing system.
What is understood by a contact sensor is a sensor that enables a statement about the contact of the gripper with an object. The quantities that can be measured at the contact surface between gripper and object can thereby be subdivided into qualitative and quantitative information. For example, spatial forces and moments as well as the attack point of the force can be qualitatively acquired. Further contact quantities such as gripping and holding forces can be determined from these quantities. The sensor employed for the acquisition of these quantities is also referred to a force sensor.
In contrast thereto, what is referred to as a tactile sensor frequently measures a purely qualitative quantity. An important qualitative information is comprised, for example, therein whether a gripper jaw of a two-jaw gripper as disclosed in [1] has contact with an object. This is often difficult to decide with exclusive employment of a force sensor given a lightweight object. The nature of the contact is another important qualitative information. A further aim is to identify the shape and surface quality of the grasped object with a tactile sensor. Further, the slipping of an object can be determined with a tactile sensor. Further, an object can be recognized, distinguished and a statement about the position and the attitude of the grasped object can be made with the tactile sensor.
Frequently employed embodiments of a contact sensor are a wire strain gauge, a piezoceramic or a semiconductor pressure sensor.
It is also known from [2] that the object sensor and the contact sensor exhibit various disadvantages. Thus, the laser distance sensor and the camera viewing system is [sic] expensive and makes [sic] high demands of an evaluation electronics. In contrast thereto, the ultrasound distance sensor in fact exhibits a low price and a great ruggedness. However, the precision of such a sensor, its susceptibility to disturbance with respect to a temperature fluctuation, an external signal or a multiple reflection and the low range of the sensor make the use thereof only conditionally possible.
Another disadvantage of the known sensors is comprised therein that a plurality of sensors are needed for the simultaneous measurement of a quantitative and qualitative state quantity. This in turn leads thereto that the structural size of such a combined sensor is larger compared to an individual sensor.
The invention is thus based on the problem of determining the relative position of two objects with respect to one another in a simple, flexible and cost-beneficial way as well as with the possibility of a miniaturization of the system.
The problem is solved by the arrangement according to patent claim 1, the set with a plurality of arrangements according to patent claim 16 as well as by the method according to patent claim 21.
The arrangement for determining a relative position of two objects with respect to one another comprises a radiation source that is configured such that it generates radiation having a characteristic radiation field. The arrangement further comprises a receiver that is configured for the reception of the radiation, and an evaluation unit coupled to the receiver with which the relative position of the objects with respect to one another can be determined from the signals received from the receiver, said signals being dependent on the reception location in the characteristic radiation field.
The following is to be understood as the characteristic radiation field of the radiation source:
The radiation source outputs radiation and thus generates a three-dimensional radiation field. The radiation has known radiation quantities at every point in the three-dimensional radiation field, whereby the respective point can be unambiguously determined from the known radiation quantities.
The set with a plurality of arrangements for determining a relative position of two objects with respect to one another comprises a plurality of arrangements, whereby each arrangement comprises a radiation source that is configured such that it generates radiation with a characteristic radiation field and comprises a receiver that os configured for the reception of the radiation. The set with a plurality of arrangements further comprises at least one evaluation unit coupled to the receivers with which the relative position of the objects with respect to one another from the signals received from the receiver, said signals being dependent on the reception location in the characteristic radiation field.
The following steps are implemented in the method for determining a relative position of two objects with respect to one another:
a) a radiation source generates radiation with a characteristic radiation field;
b) a receiver measures radiation quantities of the radiation;
c) the receiver transmits signals, which are dependent of the measured radiation quantities in the characteristic radiation field, to the evaluation unit;
d) the evaluation unit determines the relative position of the objects with respect to one another dependent on the signals received by the receiver.
A very simple method and a very simple arrangement for a position sensor are created by the invention. This position sensor makes it possible to simultaneously measure six degrees of motion freedom with only one sensor. The invention additionally permits the miniaturization of such an arrangement. Such a sensor can the be cost-beneficially produced and very flexibly utilized.
Preferred developments of the invention derive from the dependent claims.
The arrangement of radiation source and receiver can ensue in various ways. It has proven advantageous in view of a further simplification of the invention that either the first object carries the radiation source and the second object carries the receiver or the first object carries both the radiation source as well as the receiver. In this case, the second object is equipped with a reflector.
The characteristic radiation field is preferably an asymmetrical radiation field. Simplifications in the interpretation of the radiation quantities derive as a result thereof.
It is provided in a further development that the two objects are connected to one another by an elastic connection having a predetermined stiffness. A force acting between the objects and/or a moment acting between the objects can thus be additionally identified.
For cost reasons, it is advantageous to employ the following elastic connections:
a) at least one spring element;
b) silicone;
c) cellular material.
When the elastic connection exhibits electrical conductivity, then it is possible to also use the elastic connection as an electrical conductor.
In order to determine the force acting between the objects and/or the moment acting between the objects with such an arrangement, the evaluation unit coupled to the receiver is configured such in one embodiment that forces and/or moments can be determined from the signals received by the receiver dependent on the stiffness of the connection. The embodiment of such a force sensor works according to the following method:
a) the radiation source generates the characteristic radiation field in a first relative position of the objects with respect to one another;
b) the receiver measures the radiation quantities of the radiation in the first position;
c) the receiver transmits the signals, which are dependent on the measured radiation quantities in the characteristic radiation field, to the evaluation unit;
d) the evaluation unit determines the first relative position of the objects with respect to one another dependent on the signals received by the receiver;
e) a relative positional change of the two object with respect to one another is produced;
f) the radiation source generates the characteristic radiation field in a second relative position of the objects with respect to one another;
g) the receiver measures the radiation quantities of the radiation in the second position;
h) the receiver transmits the signals, which are dependent on the measured radiation quantities in the characteristic radiation field, to the evaluation unit;
i) the evaluation unit determines the second relative position of the objects with respect to one another dependent on the signals received by the receiver;
j) the force causing the positional change and/or the moment causing the positional change is calculated dependent on the stiffness of the connection of the two objects.
Such an embodiment makes it possible to implement a multi-dimensional force and/or moment measurement with only one sensor.
It has proven advantageous in view of the production costs to employ LED elements for the radiation source and/or phototransistors for the receiver.
The arrangement of the LED elements and of the phototransistors preferably ensues in two different forms. The calculating outlay for determining the relative position or, respectively, the force and/or the moment between the two objects is reduced as a result thereof. When the relative position of the two objects with respect to one another is defined in Cartesian coordinates, it is expedient to arrange the LED elements and the phototransistors equidistantly right-angled. This is particularly suited for the measurement of a relative displacement of the two objects with respect to one another along the translation directions. For the positional determination of the two objects with respect to one another in cylindrical coordinates, it is advantageous to arranged the LED elements circularly and the photo-transistors cross-shaped.
It is also advantageous in one development that the radiation source generates electromagnetic radiation like visible light. The receiver measures the light intensity as physical radiation quantity. Simple and cost-beneficial, light-sensitive, standard photoreceptors can then be utilized.
It is provided in a further development that the force sensor is employed for determining the gripping force and/or the gripping moment of a gripper. It is advantageous in view of the precision of the measurement to respectively mount a plurality of force sensors next to one another on insides of a two-jaw gripper that are directed toward one another. Given this embodiment, the evaluation unit is correspondingly configured such that
a) the position of a third object grasped with the gripper
b) and the gripping force acting on the third object and/or the gripping moment acting on the third object can be determined.